The present invention relates to cryogenic apparatus such as superconducting electromagnet assemblies of magnetic resonance imaging equipment; and more particularly to cooling systems used in such apparatus.
Magnetic resonance imaging (MRI) systems generate extremely intense magnetic fields within an imaging volume defined by a tubular magnet assembly. In order to produce the intense magnetic fields with minimum power consumption, superconducting electromagnetic coils are employed in the assembly. However, conventional superconducting materials must be cooled to approximately ten degrees Kelvin or less before their resistivity is reduced to a point where they become superconducting. As a consequence, the superconducting coils typically are enclosed in a cryostat filled with liquid helium to maintain the coils at a low temperature.
An alternative system has been proposed in which a cryogenic refrigeration unit cools the magnet assembly to the superconducting temperature, thereby eliminating the need for liquid helium. The refrigeration unit is mechanically coupled to the magnet assembly by thermal conductors which are capable of being disconnected and reconnected for routine maintenance of the magnetic resonance imaging system. Therefore, thermal connectors are required which can be repeatedly broken and yet when attached, provide relatively high contact thermal conductance.
Contact thermal conductance is defined as the heat transfer rate divided by the contact surface area and temperature difference across an interface. A small temperature difference between different parts of the connector results in a large contact thermal conductance. Generally the science of contact conductance is concerned with maximizing the conductance and obtaining consistent, high levels of conductance. One of the most hostile environments in which to form a good thermal contact is in a vacuum which is the very environment necessary for superconducting magnet hardware and cryogenic systems in general. When metal-to-metal contacts are cycled in a vacuum, there is always the likelihood that local welding of the materials will occur as well as other forms of gross interface damage.
The present inventors considered using thermal connectors comprising a pair of copper fittings with mating surfaces machined to a high tolerance flatness and parallelism. A wafer of relatively soft thermally conductive material, such as indium, was clamped between the fittings which were held together by bolts. Under the clamping pressure, the indium wafer deformed into asperities in the surfaces of the fittings, thereby making a thermal, metal-to-metal contact. The soft metal wafers were kept thin because of their relative expense and lower thermal conductance than the copper components of the thermal conductor.
Even with the indium interface, a good metal-to-metal contact can be obtained only with very smooth, parallel surfaces. Contact conductance of this type of connection was found generally to be a function of the total force on the interface, but independent of interface area. The local metal-to-metal contact area is fixed by the plastic flow point of the soft metal wafer and an increase in the overall area of the interface did not increase the local contact area. Increasing the interface clamping force beyond a given amount did not result in a further increase in conductance. This effect results from the soft metal being hydrostatically constrained by its own frictional resistance from flowing between contacting regions and non-contacting regions of the interface. Under this hydrostatic constraint, materials bear stress well in excess of their plastic flow strengths.
In addition, the planar surfaces of the connector components, tend to oxidize during periods when the connection is broken and exposed to air. The oxide coating reduces the thermal conduction upon reconnection.
As a result, it is desirable to find a mechanism, or structure, which will alleviate these limitations of the thermal connectors.